As one kind of a scheme of sharing data among a plurality of communication terminals, there is a scheme in which individual communication terminals share data in a distributed manner without using a central management apparatus such as a server.
For example, a technology, in which when communication terminals have approached to one another by movement and the like, they exchange their own information and perform synchronization of possession information by using an Epidemic scheme, has been proposed as a first associated technology associated with the present invention (for example, see Non-Patent Literature 1). In the Epidemic scheme, when a communication terminal in a network detects another communication terminal, the communication terminal exchanges its own summary vector (hereinafter, written as SV) and recognizes the contents of SV of the other communication terminal. The SV is a message which has summarized possession information of a terminal and is notified to another communication terminal. Based on the received SV, the communication terminal investigates data not possessed therein and attempts to acquire the data. For example, as illustrated in FIG. 17, a terminal T1 and a terminal T2 having approached to each other by movement mutually exchange SV. Next, the terminal T2 confirms whether there is data not possessed therein by comparing the SV of the terminal T1 with its own SV. In the example of FIG. 17, since the terminal T2 does not possess data B, the terminal T2 transmits an acquisition request of the data B to the terminal T1 and acquires the data B from the terminal T1. The aforementioned operation is repeated by all terminals in a network, so that these terminals allow possession information among them to coincide with one another.
Furthermore, as a second associated technology associated with the present invention, the following technology has been proposed (for example, see Patent Literature 1). First, a communication terminal mounted to a vehicle detects the position, relative speed and the like of a communication terminal (a counterpart terminal) mounted to another vehicle and estimates a communicable time before stating communication with the counterpart terminal by short range wireless communication. Secondly, as the communicable time is further short according to the estimation results, a further light communication protocol is selected. In detail, in the second associated technology associated with the present invention, when the communicable time is short, a communication protocol (for example, ALOHA), in which a time required for the transmission/reception of parts (an access control packet, a header part and the like) other than information (a payload) of data itself is short, is selected as the aforementioned light communication protocol. Furthermore, when the communicable time is long, a communication protocol such as CSMA/CA (Carrier Sense Multiple Access with Collision) is selected.